This invention relates to echo cancellation in telephone systems and, in particular, to cancellation of acoustic and electronic echoes in telephone systems.
There are two kinds of echo in a telephone system, an acoustic echo between an earphone or a speaker and a microphone and electrical echo generated in the switched network for routing a call between stations. In a handset, acoustic echo is typically not much of a problem. In speaker phones, where several people huddle around a microphone and loudspeaker, acoustic feedback is much more of a problem. Hybrid circuits (two-wire to four-wire transformers) located at terminal exchanges or in remote subscriber stages of a fixed network are the principal sources of electrical echo.
The distance that a signal travels causes a minimum delay. Digital calling apparatus further delays a signal in the digitizing process and in the batch (packet) mode that signals are often handled. Using a satellite relay can add considerably to the delay; a minimum of 250 milliseconds each way.
An echo is perceived if a delay is greater than approximately twenty milliseconds. Digital packet transmission through a satellite can produce a delay in excess of 600 milliseconds. Modern network equipment is incapable of handling a delay longer than about 100 milliseconds. Acoustic delays, such as reverberations in a room, can be much longer, up to 1,500 milliseconds.
In the prior art, echo canceling apparatus operates by subtracting a delayed or reconstructed original signal, or a contrived signal, from a composite signal (original sound plus echo) present in the line. The echo canceling apparatus can be either digital or analog. If digital, echo paths having very long propagation delays can be accommodated.
In a constantly changing environment, such as a telephone system, both electronic delays and acoustic delays can change during a call. In the prior art, the settings for an echo canceling circuit are not changed during a call, largely due to a long convergence time in the circuitry for finding and canceling an echo. Changing settings during a call would cause noticeable distortion in the sound, somewhat like a magnetic tape recording when the tape is deformed.
The sampling rate of analog to digital (A/D) converters in telephone systems is typically 8,000 samples per second. This number was chosen because of the relatively narrow bandwidth of a telephone system, 300-3,400 Hz, and because of the speed limitations of digital signal processing (DSP) devices. At 8,000 samples per second, the samples are separated by 125 microseconds and a 3.4 kilohertz signal is sampled only 2.3 times per cycle. This is not particularly good resolution.
In order to increase resolution, one must increase the number of samples, which causes a corresponding increase in the number of storage sites. The number of storage sites is limited by the cost of manufacturing suitable integrated circuits and the complexity of addressing the sites in real time. It is desired to have echo canceling apparatus that is both effective and compact in order to fit with handsets, speaker phones, cellular telephones, and the like.
In an analog system, the signals are not converted to digital data, which simplifies the circuitry. However, the storage time for the samples is presently limited by the characteristics of the storage node to approximately one half second without some sort of refreshing. For longer storage times, A/D conversion and memory storage are necessary.
A large number of storage sites adversely affects the time for the system to lock onto the delay, referred to herein as convergence. In a constantly changing environment, such as a telephone, system delays can change during a call and acoustic delays can change during a call because a person moves about a room. In the prior art, the settings for an echo canceling circuit are not changed during a call, largely due to a long convergence time.
In view of the foregoing, it is therefore an object of the invention to provide improved apparatus for canceling acoustic echoes and electrical echoes in telephone apparatus.
Another object of the invention is to provide echo canceling apparatus that converges quickly on the echo.
A further object of the invention is to provide acoustic echo canceling apparatus capable of delaying a signal for one second or more.
Another object of the invention is to provide echo canceling apparatus that can match phase within less than one degree using relatively few delay elements.
A further object of the invention is to provide echo cancellation apparatus that can adjust to changes in echo during a telephone call.
The foregoing objects are achieved by this invention in which a transmitted signal is coupled through a coarse delay and a fine delay to one input of a correlator. Another input of the correlator is coupled to a received signal. The delays are adjusted for maximum correlation and the output of the fine delay is subtracted from the received signal to reduce or eliminate an echo. The delays operate by sampling the signal and storing the samples at successive storage sites. The storage sites are read a predetermined time later, producing the delay. The read operation for the coarse echo includes three readings from separate sites and using the correlation products to indicate the magnitude and direction for changing the delay.